Add filename sanitizers for windows filenames

* src/common.cpp (sanitizeFileName, validateWindowsFileName)
(windowsDeviceSubDirPattern, windowsDeviceNoSubDirPattern): New.
* src/common.h: Update accordingly.

This fixes working with attachments which contain invalid
characters in their name. Previously getTmpOutfile would have
ended up with just the completely generic name attachment. Now
we remove the invalid characters from the filename with
sanitizeFileName.

• GnuPG-Bug-Id: T6864